Triple isotope effects accompanying evaporation of water: new insights from laboratory experiments

<p>Stable isotopes of hydrogen and oxygen (<sup>2</sup>H and <sup>18</sup>O) are often used for quantification of water budgets of lakes and other surface water bodies, in particular for the assessment of underground components of those budgets [1]. Recent advances in laser spectroscopy enabled simultaneous analyses of <sup>2</sup>H, <sup>18</sup>O and <sup>17</sup>O content in water, with measurement uncertainties comparable (δ<sup>18</sup>O) or surpassing (δ<sup>2</sup>H) those routinely achieved by off-line sample preparation methods combined with conventional IRMS technique [2]. This open up the doors for improving reliability of isotope-aided budgets of surface water bodies by adding third isotope tracer (<sup>17</sup>O). This, however, requires adequate information on triple isotope effects accompanying evaporation of water, in particular the kinetic isotope effect related to evaporation of <sup>1</sup>H<sub>2</sub><sup>17</sup>O isotopologue.</p><p>Here we present the results of dedicated laboratory experiments aimed at quantification of triple isotope effects accompanying evaporation of water under fully developed diffusive sublayer [3]. Identical containers with predefined mass of water of known isotopic composition were placed in an isolated chamber with controlled atmosphere during the experiment (temperature, relative humidity). The chamber was flushed with synthetic air. At regular time intervals (approximately one week) containers were removed one by one from the chamber, the remaining mass of water in the removed container was determined gravimetrically, and stored for subsequent isotope analyses. The flow rate was adjusted at each step of the process to keep humidity inside the chamber constant. Evaporation continued until approximately half of the initial mass of water was removed from the containers. The experiment was repeated under diiferent conditions inside the chamber (two different temperatures and three different values of relative humidty).</p><p>The results of the experiments were interpreted in the framework of Craig-Gordon model of evaporation [3]. It turned out that the assumption often used in the description of isotopic effects accompanying evaporation that liquid phase is isotopically homogeneous during the process, leads to conflicting results for three isotope systems in use. However, if surface enrichment of the liquid phase, different for each heavy isotopologue (<sup>1</sup>H<sup>2</sup>H<sup>16</sup>O, <sup>1</sup>H<sub>2</sub><sup>18</sup>O, <sup>1</sup>H<sub>2</sub><sup>17</sup>O) is included in the model, consistent results for all three isotopes can be achieved, with calculated kinetic fractionation factor for <sup>1</sup>H<sub>2</sub><sup>17</sup>O isotopologue equal 14.76 ± 0.48 ‰,. This value agrees, within the quoted uncertainty, with the value of 14.60 ± 0.30 ‰ obtained by Barkan and Luz [4].  </p><p>Acknowledgements: The presented work was supported by National Science Centre (research grant No. 2016/23/B/ST10/00909) and by the Ministry of Science and Higher Education (project no. 16.16.220.842 B02)</p><p>References:<br>[1]   Rozanski K. Froehlich K. Mook WG. Technical Documents in Hydrology, No. 39, Vol. III, UNESCO, Paris, 2001 117 pp.<br>[2]   Pierchala A, Rozanski K, Dulinski M, Gorczyca Z, Marzec M, Czub R, Isotopes in Environmental and Health Studies, 2019 (55) 290-307.<br>[3]   Horita, J. Rozanski K. Cohen S. 2007. Isotopes in Environmental and Health Studies, 2007 (44) 23-49.<br>[4]   Barkan E. Luz B. Rapid Commun. Mass Spectrom., 2007(21) 2999-3005.</p>

High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates

<p>Oxygen isotopes are a widely used tracer in the field of paleoceanography and provide unique information on mineral formation and environmental conditions. Carbonate sediments record a shift in δ<sup>18</sup>O of 10 to 15‰ from the Archean towards higher values in the Phanerozoic. Three different scenarios are suggested to explain this observation: (I) hot Archean oceans, (II) depletion of <sup>18</sup>O in Archean oceans compared to present day and (III) diagenetic alteration of the primary isotopic signature [1]. Recent advances in high-resolution gas source isotope ratio mass spectrometry provide a new tool that may allow to decipher the origin of this isotopic shift observed in the early rock record. We performed high-precision <sup>18</sup>O/<sup>16</sup>O and <sup>17</sup>O/<sup>16</sup>O measurements on oxygen ion fragments (<sup>16</sup>O<sup>+</sup>, <sup>17</sup>O<sup>+</sup>, <sup>18</sup>O<sup>+</sup>) generated in the ion source from CO<sub>2</sub> gas [2]. Isobaric interferences on m/z=17 (<sup>16</sup>OH<sup>+</sup>) and m/z=18 (H<sub>2</sub><sup>16</sup>O<sup>+</sup>) are separated by means of high mass resolution. The CO<sub>2</sub> gas is first liberated from carbonate samples by orthophosphoric acid digestion and then analyzed on a <em>Thermo Scientific Ultra</em> dual-inlet gas source isotope ratio mass spectrometer [3]. By adding the dimension of <sup>17</sup>O/<sup>16</sup>O to the classical<sup> 18</sup>O/<sup>16</sup>O system, equilibrium trajectories of carbonates that are defined by the equilibrium fractionation factor (<sup>18</sup>a<sub>eq</sub>) and the triple isotope fractionation exponent (θ) can be predicted as a function of temperature. Minerals that were altered by or formed in meteoric water can be distinguished from those that precipitated in equilibrium with ambient sea water. Therefore, triple oxygen isotope analysis of carbonates does not only hold the potential for a new single-phase paleothermometer, but may also be used to trace the origin of carbonates. Here, we present high-precision triple oxygen isotope data for carbonates from the Pilbara and the Kaapvaal cratons that cover nearly one billion years from the Paleoarchean to the Paleoproterozoic. Marine carbonates from the Phanerozoic complement the dataset. The carbonates were formed in different marine settings, from shallow marine stromatolites to carbonates grown in the interstitial space of basaltic pillows. Phanerozoic carbonates record equilibrium conditions with modern sea water at moderate temperatures. The majority of Precambrian carbonates plot below the predicted equilibrium curve in the δ’<sup>18</sup>O-Δ‘<sup>17</sup>O space and do not reflect equilibrium conditions with modern sea water at elevated temperatures that were proposed for the Archean oceans. Modeling the triple oxygen isotope composition of carbonates in equilibrium with sea water, that is depleted in <sup>18</sup>O also cannot explain the observed isotopic shift. Further modeling of post-depositional alteration suggests that most carbonates interacted and re-equilibrated with meteoric waters at variable water-rock ratios and temperatures.</p><p>[1] Shields and Veizer, 2002, Geochem., Geophy., Geosyst., 10.1029/2001GC000266<br>[2] Getachew et al., 2019, Rapid Commun. Mass. Spectrom., 10.1002/rcm.847<br>[3] Eiler et al., 2013, Int. J. Mass. Spectrom., 335, 45-56.</p>

Determination of the Ni isotope fractionation in microfossils embedded in the aragonite phase

<p>Stable nickel isotopes are known to fractionate by biological processes and their measurements can be important biomarker. In searches for ancient fossilised materials such as microbial cells, the Ni isotope fractionation record can be preserved after death and fossilization of microstructures. Typically, transition metal isotopes in microfossils are difficult to measure accurately because of their low concentration in the fossil. Furthermore, microsized fossil structures  are difficult to isolate from the host phase. Thus, the measurement of their chemical composition can be conducted only by a few  analytical methods. We have applied femtosecond-laser ablation/ionisation time-of-flight mass spectrometry (LIMS) to measure chemical composition of the fossilised material embedded in the aragonite phase and accurately derive the Ni isotopic fractionation pattern. High resolution depth profiling method was applied to isolate fossilised material composition from the host phase. The mass peak intensity correlation and peak integration methods were subsequently applied to derive isotope concentrations. The accuracies and precision in permill level or better of the isotope values were achieved. For comparison the studies of Ni isotopes were conducted on inorganic samples. The instrument used in the studies is a miniature mass analyser developed for space research holding promisses that differentiation between abiotic and biogenic microstructures in rocks can be studied also in situ on the surfaces of Solar System bodies.</p><p>References</p><p>1. U. Rohner et al., Meas. Sci. Technol. 14 (2003) 2159–2164</p><p>2. A. Riedo et al., JAAS, 28:1256–1269, 2013</p><p>3. A. Neubeck et al., Int. J. Astrobiology, 15, 133-146, 2016</p><p>4. M. Tulej et al., Astrobiology, 2015, DOI: 10.1089/ast.2015.1304;JAAS,33(8):1292-1303, 2018</p><p>5. S. Meyer et al., J. Mass Spectrom. 2017, DOI: org/10.1002/jms.3964   </p><p>6. R. Wisendanger et al., J. Chemometrics, 2018, DOI: 10.1002/cem.3081</p><p>7. V. Grimaudo et al., Anal. Chem., 2018, DOI: 10.1021/acs.analchem.7b05313</p><p> </p><div> </div>

In Silico Investigation of Mitragynine and 7-Hydroxymitragynine Metabolism

Objective: Mitragynine is the main active compound of Mitragyna speciose (Kratom in Thai). The understanding of mitragynine derivative metabolism in human body is required to develop effective detection techniques in case of drug abuse or establish an appropriate dosage in case of medicinal uses. This in silico study is based upon in vivo results in rat and human by Philipp et al. (J. Mass Spectrom., 2009, 44, 1249.) Results: The gas-phase structures of mitragynine, 7-hydroxymitragynine and their metabolites were obtained by quantum chemical method at B3LYP/6-311++G(d,p) level. Results in terms of standard Gibbs energies of reaction for all metabolic pathways are reported with solvation energy from SMD model. We found that 7-hydroxy substitution leads to changes in reactivity in comparison to mitragynine: position 17 is more reactive towards demethylation and conjugation to a glucuronide and position 9 is less reactive towards conjugation to a glucuronide. Despite the changes, position 9 is the most reactive for demethylation and position 17 is the most reactive for conjugation to a glucuronide for both mitragynine and 7-hydroxymitragynine. Our results suggest that 7-hydroxy substitution could lead to different metabolic pathways and raise an important question for further experimental studies of this more potent derivative.

In Silico Investigation of Mitragynine and 7-Hydroxymitragynine Metabolism

Objective: Mitragynine is the main active compound of Mitragyna speciose (Kratom in Thai). The understanding of mitragynine derivative metabolism in human body is required to develop effective detection techniques in case of drug abuse or establish an appropriate dosage in case of medicinal uses. This in silico study is based upon in vivo results in rat and human by Philipp et al. (J. Mass Spectrom., 2009, 44, 1249.) Results: The gas-phase structures of mitragynine, 7-hydroxymitragynine and their metabolites were obtained by quantum chemical method at B3LYP/6-311++G(d,p) level. Results in terms of standard Gibbs energies of reaction for all metabolic pathways are reported with solvation energy from SMD model. We found that 7-hydroxy substitution leads to changes in reactivity in comparison to mitragynine: position 17 is more reactive towards demethylation and conjugation to a glucuronide and position 9 is less reactive towards conjugation to a glucuronide. Despite the changes, position 9 is the most reactive for demethylation and position 17 is the most reactive for conjugation to a glucuronide for both mitragynine and 7-hydroxymitragynine. Our results suggest that 7-hydroxy substitution could lead to different metabolic pathways and raise an important question for further experimental studies of this more potent derivative.

Extended kinetic method and RRKM modeling to reinvestigate proline’s proton affinity and approach the meaning of effective temperature

Proline proton affinity PA(Pro) was previously measured by extended kinetic methods with several amines as reference bases using a triple quadrupole mass spectrometer ( J Mass Spectrom 2005; 40: 1300). The measured value of 947.5 ± 5 kJ.mol−1 differs by more than 10 kJ.mol−1 from previous reported experimental or calculated values. This difference may be explained in part by the existence of relatively large entropy difference between the two dissociation channels (ΔΔS‡avg = 31 ± 10 J.mol−1.K−1) and by the inaccuracy of the amines proton affinity used as reference bases. In the present work, these experimental measurements were reinvestigated by RRKM modeling using MassKinetics software. From this modeling, a new PA value of 944.5 ± 5 kJ.mol−1 and a ΔΔS‡avg(600K) value of 33 ± 10 J.mol−1.K−1 are determined. However, the difference between experiment and recent theoretical calculations remains large (10 kJ.mol−1). These RRKM simulations allow also accessing to the effective temperature parameter (T eff) and to discuss the meaning of this term. As previously reported, T eff mainly depends on the internal energy and on the decomposition time as well. It also depends on the critical energies and on the transition state. Considering the entrance of the collision cell as a new ion source, T eff is finally shown to be close to a characteristic temperature (T char).